For a d2, flip a coin. For a d3, roll a d6 and if you get any result above 3, subtract 3 (so 4 is 1, 5 is 2, 6 is 3). For negative modifiers, subtract the result from whatever damage you rolled, and if the total is equal to or less than 0, the damage inflicted is 1 because that's the minimum damage an attack can do (though damage reduction can later reduce this to 0).

For your specific example, if the monster only gets one attack (it's a surprise round, or it has moved and thus doesn't have the option to full attack), you roll d20+4 and compared it to the target's armor class to see if it hits, then check damage if it does hit (though since 1d2-1 can only ever do 1 damage, you might as well not bother rolling). If the creature does nothing but attack all round (exceptions: 5' step and free actions), you do that twice, and also roll d20-1 and check its bite damage (likely another 1 but not necessarily, so roll this one) if that connects.

It means the attack is a melee attack. It does not draw attacks of opportunity for being used in threatened space, and it cannot hit somebody on the other end of a football field. It also means that to determine whether it hits the target's armor class, when you roll the d20, you add 4 to the result before comparing it to the target's armor class.

I'd recommend reading the PHB before coming here to post. I mean, it's useful. They say it contains the rules of the game. Also, You might want to have a look at the DMG, as it's a purportedly useful Guide to aspiring Dungeon Masters. As a last note, let me advise you to peruse the MM. This Manual provides Monsters, and rules on how to use them. Seriously, reading these books might help make you a better DM at D&D. It worked for me!

OK, a brief outline of the combat system: Combat is divided up into turns or rounds. A round of combat is about six seconds long, and in that time you have several types of action:

Immediate Action - something you can do at ANY TIME, and consists of something very simple and fast. Immediate actions can be taken out of initiative order, all other actions you take have to take only when it is 'your turn'.

Free Action - again, something very quick and simple, like dropping what you are holding, letting go of a rope, turning on the spot. You can take any number of free actions in a round.

Swift Action - usually the activation of a power, or the casting of a very quick spell, this takes 'no time' like a free action, but you can only perform one in a round.

Move Action - move actions are simple actions that nonetheless consume time. A move action is sometimes something that you while also doing a standard action - like running into a room (move action) and hitting somebody (standard action). Drawing or sheathing a weapon are move actions, as is (not surprisingly) moving. Move actions don't generally take a lot of mental effort or coordination.

Standard Action - swing a sword, cast a normal spell, climb over an obstacle, swing on a chandelier - these are all standard actions. This takes the lion's share of the combat round, at least in terms of concentration.

Full Action - this is when you concentrate on doing ONE THING for the entire combat round. Some spells have long casting times, or really laying into the target in front of you with everything you've got.

This listing gives you the attack a creature would make as a standard action - that is what it will do if it moves and fights, or makes an attack of opportunity. Then there is the Full Attack, which is what it deals out if it has the chance to just maul you (a full attack is a full round action).

What it means?

To make an attack, you roll a d20 and add the attack bonus. If this beats the target's Armour Class (AC) then you hit, if not, you miss. A roll of a natural 20 on the dice is always a hit, regardless of attack bonus and AC.

If you hit, you do damage. Damage is expressed as a dice and a modifier, such as '1d6+2' which means you roll a six sided dice and add two to the result. So 1d3-1 damage is a 'three sided' dice (actually you roll a normal six-sided dice and half the result, rounding halves up) with one point subtracted, so it may do 0, 1 or 2 points of damage.

'2 claws +4 melee (1d2-1) and bite -1 melee (1d3-1)' means that this creature gets two attacks at +4 attack bonus, each inflicting 1d2-1 damage, followed by one attack at -1 attack bonus which inflicts 1d3-1 damage, as a full round attack.